The covers of golf balls are generally made from a variety of materials, such as balata or ionomer resins such as SURLYN.RTM. and IOTEK.RTM.. Balata, which is a natural or synthetic trans-polyisoprene rubber, is the softest of these cover materials. Balata covered balls are favored by the more highly skilled golfers because the softness of the cover allows the player to achieve spin rates sufficient to more precisely control ball direction and distance, particularly on shorter shots.
However, balata covered balls are expensive and less durable as compared to the other cover materials. In particular, balata covered balls are subject to nicks or cuts as a result of a mis-swung golf club or due to landing on cart paths, etc. and/or contact with rocks, trees, etc. Such nicks or cuts detract from the flight characteristics of such balls, rendering them of little use. Accordingly, cover compositions have been developed in an attempt to provide balls with spin rates and a feel approaching those of balata covered balls, while also providing a golf ball with a higher durability and overall distance.
Ionomer resins have, to a large extent, replaced balata as a cover stock material. Chemically, ionomer resins are a copolymer of an olefin and an alpha, beta ethylenically-unsaturated carboxylic acid having 10-90% of the carboxylic acid groups neutralized by a metal ion. See U.S. Pat. No. 3,264,272. Commercially available ionomer resins include, for example, copolymers of ethylene and methacrylic or acrylic acid. These are sold by E.I. DuPont de Nemours and Co. under the trademark "SURLYN.RTM." and by the Exxon Corporation under the trademark "ESCOR.RTM." and the trademark "IOTEK.RTM.". These ionomer resins are distinguished by the type of metal ion, the amount of acid, and the degree of neutralization. Also, Chevron Chemical Co. sells a family of ionomers produced from ethylene acrylate-based copolymers under the trademark "IMAC.RTM.".
U.S. Pat. Nos. 3,454,280, 3,819,768, 4,323,247, 4,526,375, 4,884,814, and 4,911,451 all relate to the use of SURLYN.RTM.-type compositions in golf ball covers. However, while SURLYN.RTM. covered golf balls as described in the preceding patents possess virtually cutproof covers, they have inferior spin and feel properties as compared to balata covered balls.
In 1986, DuPont introduced two new classes of ionomer resins. One was a sodium and zinc ionomer resin having a low flexural modulus. DuPont suggested using and blending the same with other ionomer resins for making a golf ball cover. Golf ball covers made from these low flexural modulus ionomer resins have improved spin and feel characteristics but relatively low velocity. The other was a lithium ionomer resin which was a copolymer of ethylene and methacrylic acid. These lithium ionomer resins have a very high flexural modulus, typically about 60,000 psi (415 MPa). DuPont suggested that lithium ionomer resins could be used to produce a golf ball cover which would be more cut resistant and harder than a cover made with either sodium or zinc ionomer resins. DuPont also suggested that a golf ball having a cover made from a lithium ionomer resin would go farther, have a higher coefficient of restitution and be less prone to cutting (i.e., more durable) than a golf ball made from other known ionomer resins such as sodium and zinc ionomer resins and blends thereof. DuPont further suggested that lithium ionomer resins could be used in blends with other ionomer resins where they can impart better cut resistance to those other resins.
"The Rules of Golf" by the USGA dictates that no golf ball shall have an initial velocity that exceeds 255 feet (78 m) per second, i.e., 250 feet (76 m) per second with a 2% tolerance when impacted by the USGA test machine under specified conditions. Golf balls with covers made from ionomer resins with a low flexural modulus are significantly below this maximum and, as should be appreciated, all golf ball manufacturers strive to come as close as possible to this limit.
In various attempts to produce an ideal golf ball, the golf industry has blended hard ionomer resins (i.e., those ionomer resins having a hardness of about 60 to about 70 on the Shore D scale as measured in accordance with ASTM method D-2240) with a number of softer polymeric materials, such as softer polyurethanes (see, e.g., U.S. Pat. No. 4,674,751 to Molitor et al.). However, the blends of the hard ionomer resins with the softer polymeric materials have generally been unsatisfactory in that these balls exhibit numerous processing problems. In addition, the balls produced by such a combination are usually short on distance.
In addition, various hard-soft ionomer blends, that is, mixtures of ionomer resins which are significantly different in hardness and/or flexural modulus, have been attempted. U.S. Pat. No. 4,884,814 discloses the blending of various hard methacrylic based ionomer resins with similar or larger quantities of one or more "soft" ionomer methacrylic acid based ionomer resins (i.e., those ionomer resins having a hardness from about 25 to 40 as measured on the Shore D scale) to produce relatively low modulus golf ball cover compositions that are not only softer than the prior art hard ionomer covers but also exhibit a sufficient degree of durability for repetitive play. These relatively low modulus cover compositions were generally comprised of from about 25 to 70% of hard ionomer resins and from about 30 to 75% of soft ionomer resins.
U.S. Pat. No. 5,324,783 discloses golf ball cover compositions comprising a blend of a relatively large amount, e.g., 70-90 wt. %, of hard ionomer resins with a relatively low amount, e.g., 10 to about 25-30 wt. %, of soft ionomers. The hard ionomers are sodium or zinc salts of a copolymer of an olefin having from 2 to 8 carbon atoms and an unsaturated monocarboxylic acid having from 3 to 8 carbon atoms. The soft ionomer is a sodium or a zinc salt of a terpolymer of an olefin having from 2 to 8 carbon atoms, methacrylic acid and an unsaturated monomer of the acrylate ester class having from 1 to 21 carbon atoms.
In order to approximate the characteristics of balata covered balls at lower cost, the art has developed balls having a variety of cover compositions. There are more than fifty commercial grades of ionomers available from DuPont and Exxon with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e., relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcements, etc. As noted above, these prior art compositions have a considerably higher cut resistance and durability as compared to balata covered balls. A great deal of research continues in order to develop golf ball cover compositions exhibiting not only improved impact resistance and carrying distance properties produced by the "hard" ionomeric resins, but also the playability (i.e. "spin") characteristics previously associated with the "soft" balata covers, properties which are still desired by the more skilled golfer.
Polyurethane has also been recognized as a useful material for golf ball covers since as early as about 1960. U.S. Pat. No. 3,147,324, filed Oct. 20, 1960, is directed to a method of making a golf ball having a polyurethane cover. This patent disclosed an improved method of covering a golf ball with a liquid urethane polymer which could be applied, set and cured at room temperature or at a temperature that would not damage the tensioned windings of the golf ball center. The curing agents disclosed were diamines, polyols or air moisture. Polyurethane covered golf balls were durable and did not cut as balata balls did while at the same time maintaining the feel of a balata ball.
In the ensuing years following 1960, various companies investigated the usefulness of polyurethane as a golf ball cover material in limited ways. U.S. Pat. No. 4,123,061 issued Oct. 31, 1978 teaches that a golf ball can be made from a polyurethane prepolymer of polyether and a curing agent, such as a trifunctional polyol, a tetrafunctional polyol or a diamine. U.S. Pat. No. 5,334,673 issued Aug. 2, 1994 also discloses polyurethane covered golf balls made from a composition of polyurethane prepolymer and a slow-reacting amine curing agent and/or a difunctional glycol.
The first commercially successful polyurethane covered golf ball appeared on the market with the introduction of Titleist's PROFESSIONAL golf ball in 1993. The principal reason for the delay in bringing polyurethane composition golf ball covers on the market was that it was a daunting engineering task to apply a coating of polyurethane composition to a golf ball core to form a golf ball cover having a uniform thickness.
In particular, the difficulty resided in centering a golf ball core in an amount of polyurethane that was sufficiently cured to keep the core centered while at the same time being insufficiently cured so that the cover material could be molded around the core. This problem was essentially solved and a commercially successful polyurethane covered golf ball appeared on the market with the introduction of the aforesaid PROFESSIONAL polyurethane covered golf ball in 1993. A preferred method for centering the core is disclosed in U.S. Pat. No. 5,733,428.
Unlike SURLYN golf balls, polyurethane golf balls do possess the feel of balata golf balls. However, golf ball covers made from polyurethane have not up to the present, fully matched SURLYN golf balls in respect of resilience or the rebound or damping behavior of the golf ball cover which is a function of the initial velocity of a golf ball after impact with a golf club.
Polyureas have also been proposed as cover materials for golf balls. For instance, U.S. Pat. No. 5,484,870 issued Jan. 16, 1996 discloses a polyurea composition comprising the reaction product of an organic diisocyanate and an organic amine, each having at least two functional groups. Like polyurethanes, polyureas are not completely comparable to SURLYN golf balls with respect to resilience or the rebound or damping behavior of the golf ball cover.
To improve the playing characteristics of polyurethane covered golf balls, the use of cationic polyurethane ionomers has been proposed in U.S. Pat. No. 5,691,066. Although, the inclusion of cationic polyurethane ionomers in a golf ball cover can further increase the resilience of the ball, there seems to be a limitation in this approach due to the limited choice of suitable alkylating agents used in the processing of such cationic polyurethanes.
Therefore, none of the prior art teaches a solution to the continuing problem of providing a golf ball with an industry acceptable covers suitable for use under rigorous conditions. In view of the problem caused by the cutting, scratching, and abrasion of the covers of golf balls, there exists a need for golf ball covers that are capable of providing improved resilience, and cut and abrasion resistance and adherence without adversely affecting overall golf ball performance characteristics.